A fiber optic sensing system is a system wherein interrogating light is directed to a probe by fiber optic means, and light from the probe, with a property indicative of the second variable, is directed to photodetector means by fiber optic means.
The present invention relates to methods and devices for the measurement of physical parameters using a class of optical probes which permit the measurement of temperature by at least two physically independent optical methods, or the measurement of temperature in addition to at least another physical parameter, using a single optical probe, a single interrogating light source and a single photodetector, in conjunction with fiber optic techniques for directing interrogating to the probe and for directing light from the probe to the photodetector.
The term "light" in this application refers broadly to optical radiation, whether or not visible to the human eye. Although in the past the use of the word was restricted to visible radiation, workers in the art of fiber optics and lasers have generally adopted the broader coverage. Thus, the term "light-emitting diode" (LED) applies to a semiconductor source of infrared radiation just as well as to a source of red or yellow light. The word "LASER", which stands for Light Amplification by Stimulated Emission of Radiation, is not restricted to visible-emitting devices. In fact, most LED's and lasers are emitters of non-visible optical radiation, mostly infrared. And the "Light" in the term "Lightwave Communications" applies more often than not to optical radiation of wavelengths of about 1.3 or 1.55 micrometers (.mu.m), in the infrared region.
Like almost all inventions relating to fiber optic sensors, all patented fiber optic temperature sensors make use of previously known physical principles, properties of matter and/or optical devices. The temperature-dependent properties of matter used in fiber optic temperature sensors include (but are not limited to) temperature-dependent changes in optical birefringence, light absorption, luminescence properties, light scattering or light phase.
Long before there were any fiber optic temperature sensors, U.S. Pat. No. 2,551,650 to Urbach described the use of some photoluminescent materials for sensing temperature changes of objects on which coatings of said materials were applied.
In 1972, U.S. Pat. No. 3,639,765, described how films of some luminescent materials doped with trivalent terbium and europium ions (Tb.sup.3+ and Eu.sup.3+) could map temperature distributions on the surfaces these films were applied to. These materials required excitation with ultraviolet radiation in order to operate. The temperature at each point in the film could be determined from measuring the intensity of the green component of the luminescence (emitted by Tb.sup.3+), which decreased with increasing temperature, or the intensity of the red component (emitted by Eu.sup.3+), which increased with increasing temperature. The patent also disclosed that the decrease of the Tb.sup.3+ luminescence intensity was accompanied by a corresponding decrease of the luminescence decay time. In 1977, U.S. Pat. No. 4,061,578 introduced the technique of "spectrally-selective luminescence ratio thermometry", whereby the temperature at a sensing point is determined from the ratio of the intensities of two spectral components of the luminescence of the material (also known as "phosphor"). The main application considered in these patents was the conversion of a thermal infrared image into a visible image. The technique of spectrally-selective luminescence ratio thermometry was used later for fiber optic thermometry by Wickersheim et al. (U.S. Pat. Nos. 4,075,493 and 4,448,547), using phosphors also doped with trivalent europium ions, including phosphors previously described by Struck et al. (Struck, C. W. and Fonger W. H. Journal of Luminescence 1,2 [1970] 456-469) and by Wickersheim et al. (Wickersheim, K. A. and Buchanan, R. A. Appl. Phys. Letters 17, pp. 184-187 [1970]). A new company, Luxtron Corporation, was formed to market fiber optic temperature sensors based on Wickersheim's patents. These sensors required ultraviolet excitation and complex optical instrumentation requiring a plurality of optical filters, a plurality of lenses and mirrors, and a plurality of photodetectors, as shown in the last figure in the article by K. Wickersheim et al. "Recent Advances in Optical Temperature Measurement", Industrial Research/Development (December 1979) listed by the examiner in Form PTO-892.
An alternate technique for fiber optic temperature sensing uses the measurement of the decay time of the thermally quenched luminescence of some phosphors previously known for decades, including alkaline earth sulfides excited by ultraviolet or blue light pulses, and is the subject of U.S. Pat. Nos. 4,223,226 to Quick et al. and 4,245,507 (re-issued as U.S. Pat. No. 31,832 Feb. 12, 1985) and 4,437,772 to Samulski. In addition to requiring ultraviolet or blue excitation light, which restricts the use of the technique to relatively short fiber distances, the light pulses required expensive light sources, and the luminescence efficiency of these phosphors decreased with increasing temperature over the temperature range of operation of the sensor, which restricted their use to relatively narrow temperature ranges. For example, FIG. 4 of U.S. Pat. No. 31,832 shows that the luminescence efficiency of these phosphors decreases to one half of its initial value over a temperature range of only 29.9 kelvins.
Another example of a fiber optic temperature sensor based on the temperature-dependent decay time of thermally quenched luminescence was described by J. S. McCormack, Electronics Letters 17, pp. 630-631 (1981). It requires ultraviolet or blue light excitation, so it is only a marginal improvement over the Samulski sensors.
Yet another temperature sensor based on thermally quenched luminescence uses the temperature-dependent luminescence decay time of the so-called "R" bands of ruby, as described in the Sholes and Small references.
An improvement of the technique of spectrally-selective photolumenescent ratio thermometry is described in U.S. Pat. No. 4,376,890 to Engstrom et al., who used a a probe a fluorescent semiconductor, and an GaAlAs LED as excitation source. But the device still had the disadvantage of requiring a relatively complex electro-optical system including (but not limited to) a plurality of optical filters and a plurality of photodetectors.
At the time of filing (Aug. 6, 1982) of applicant's original application for this invention (Ser. No. 405,732, now abandoned) the only known fiber optic temperature sensors using luminescent probes required a temperature-dependent change in either the luminescence spectra of the probe or its luminescence quantum efficiency, or both. The only commercially available sensors were the ones base don the above Wickersheim patents requiring ultraviolet excitation, thermally quenched phosphors, and complex optical instrumentation requiring a plurality of optical filters, a plurality of lenses and a plurality of photodetectors, as shown by K. Wickersheim et al., Industrial Research/Development, December 1979.
Kleinerman's patent application Ser. No. 405,732 filed Aug. 6, 1982, now abandoned disclosed for the first time fiber optic temperature sensing systems using luminescent probes which did not require any temperature-dependent changes of any luminescence property for accurate temperature sensing, could be implemented with simple devices employing inexpensive and long-lived red or infrared LEDs, required no optical filters, lenses, mirrors or polarization-selective components, and could use as probes virtually any efficient luminescent material. Furthermore, the invention teaches that this technology can be applied to a special class of luminescent materials described in the section entitled "Luminescent materials with two emissive levels in thermal equilibrium" (U.S. Pat. No. 4,708,494, col. 13, line 19 to col. 14 line 53) and characterized by a luminescence decay time which decreases with increasing temperature over a wide temperature range within which the luminescence quantum efficiency is essentially constant, and that a temperature sensor based on said materials can be operated in two physically independent modes, thus allowing self-checking operation. This special kind of luminescent materials includes inorganic crystals doped with 3d transition metal ions comprising Cr.sup.3+, V.sup.2+ and Mn.sup.4+. Cr.sup.3+ -doped materials like emerald, specifically discussed in U.S. Pat. No. 4,708,494, are known to be extremely stable to high excitation powers and high thermal stress. The luminescence physics of emerald have been described by W. H. Fonger and C. W. Struck, Physical Review B, 11 pp 3251-3260 (1975), as already discussed in said U.S. Pat. No. 4,708,494 (col. 16 lines 17-63). Nothing in the paper by Fonger and Struck suggests any use for fiber optic sensors or any other application.
According to the teachings of this invention such materials, characterized by an essentially constant luminescence quantum efficiency over a wide temperature range over which their luminescence time rate of decay varies substantially, can be used as temperature probes in a plurality of methods for the optical measurement of temperature. A single probe comprised of a single such material can be interrogated in one or a plurality of physically independent methods within a single simple device using a single inexpensive and very long-lived interrogating light source, preferably a light-emitting diode (LED), a single photodetector, and without requiring the use of optical filters. Because they can be interrogated in at least two physically independent methods, these materials can be used for constructing self-checking fiber optic temperature sensors using a single probe, a single interrogating light source and a single photodetector, as claimed in claims 7 and 18 of U.S. Pat. No. 4,708,494, claims 3 and 9 of U.S. Pat. No. 5,090,818 and in this application. They can also be used to construct bi-parametric sensors, namely sensors which can measure temperature and another independent physical parameter, again using a single probe, a single interrogating light source and a single photodetector. Their actual or proposed use according to the teachings of this invention was not reported, proposed, or even hinted at, to applicant's knowledge, before applicant's filing of the parent application Ser. No. 405,732, now abandoned. At that time, the only kinds of fiber optic temperature sensors based on probe luminescence commercially available or otherwise known in the open literature required were those discussed in the section entitled "Background of the invention" of the parent application Ser. No. 405,732 filed Aug. 6, 1982, now abandoned, and its continuation application Ser. No. 608,932 filed May 14, 1984, now U.S. Pat. No. 4,708,494. All of these devices required ultraviolet or blue interrogating lights. The two main features in the new temperature sensors subject of these applications are:
(a) an optical signal from a probe generated at wavelengths .lambda..sub.1 different from the wavelength or wavelengths of the interrogating light, and with an intensity which increases with increasing temperature over the temperature range of operation of the sensor; and/or
(b) a luminescence from the same probe as in (a), with a decay time .tau. which varies substantially over a temperature range (several hundred kelvins for emerald) within which the luminescence quantum efficiency remains constant or varies only minimally.
Sensors having the above features can be implemented (as described in U.S. Pat. No. 4,708,494) with inexpensive red or near infrared LED excitation sources.
The first publicly disclosed temperature sensor using feature (a) above was the distributed temperature sensor based on the previously known phenomenon of anti-Stokes Raman scattering, and was first disclosed in a British patent application filed on 1983 (J. P. Dakin, UK Patent Application GB 2 140 554 A filed May 26, 1983). The term "anti-Stokes" means (to oversimplify) that the wavelengths .lambda..sub.1 of the emitted light are shorter than the wavelength or wavelengths of the interrogating light, as the part of the emitted light discussed and illustrated in Kleinerman's 1987 U.S. Pat. No. 4,708,494, col. 7, lines 4-28, and FIG. 2 (arrowed lines 70-71). Raman scattering from optical fibers, both Stokes and anti-Stokes, had been known in the art. Page 127 of the book "Optical Fiber Telecommunications" edited by S. E. Miller and A. G. Chynoweth (Academic Press, 1979), FIG. 5.1, shows both the Stokes and anti-Stokes bands of the Raman-scattered light of fused silica.
The first public disclosure of a fiber optic temperature sensor having feature (b) was made on September 1984 (Bosselmann, A. Reule and J. Schroder, "Fiber-optic temperature sensor using fluorescence decay time", Proc. 2d Int. Conf. on Opt. Fiber Sensors, Stuttgart, FRG, pp. 151-154, September 1984), more than two years after the date of filing of Kleinerman's 1982 parent application Ser. No. 405,732, now abandoned. In 1985 the Luxtron Corporation began marketing a fiber optic sensor (U.S. Pat. No. 4,652,143) based on a probe comprised of Mn.sup.4+ -doped magnesium fluorogermanate, a phosphor with luminescence properties previously described by G. Kemeny and C. H. Haake, J. Chem. Phys. 33, pp. 783-789 (1960). This probe, with a luminescence decay time .tau. which varies over a temperature range within which the luminescence quantum efficiency remains constant or varies only minimally, was operated according to the teachings of Kleinerman's U.S. Pat. No. 4,708,494, section entitled "Luminescent materials with two emissive levels in thermal equilibrium", col. 13, line 19 to col. 14 line 53. However, the phosphor has the disadvantage that it requires ultraviolet or violet excitation light from a Xenon flashtube (see K. A. Wickersheim and M. H. Sun, J. Microwave Power, pp. 85-94 [1987]), a high voltage source which is inefficient, expensive and relatively short-lived. A further disadvantage of the Mn.sup.4+ -based Luxtron sensors is that their temperature coefficient of luminescence decay time is lower than 1 percent per .degree. C., too low to be accurate for many applications.
The first commercially available fiber optic temperature sensor using as a probe a luminescent material with a temperature-dependent luminescence decay time and excitable with red or near infrared LEDs was announced by Wickersheim in 1991 in "Application of fiber optic thermometry to the monitoring of winding temperatures in medium and large power transformers", SPIE Proceedings Vol. 1584 (1991).